Apparatus for measuring ultrasonic power

ABSTRACT

An ultrasonic power meter is provided for measuring ultrasonic power emitted by a device ( 18 ) under test. The meter includes a casing forming a chamber ( 16 ) within which an ultrasonic absorber ( 14 ) formed from polyurethane material is located. Overlying the absorber ( 14 ) is a membrane ( 12 ) of polyvinylidene fluoride which acts as a pyroelectric detector. The meter also includes a transfer medium ( 10 ), typically water, for allowing the transfer of ultrasonic energy emitted from a device ( 18 ) under test to the meter.

The present invention relates to an ultrasonic power meter, for use, forexample, in measuring ultrasonic power being delivered by physiotherapyultrasound machines.

Ultrasonic machines of this type are widely applied within hospitals inthe United Kingdom and elsewhere for the treatment of soft-tissueinjuries. The treatment heads used are coupled to the body using awater-based gel and ultrasonic power is delivered to the clinical siteof interest.

Several equipment surveys carried out world-wide has long indicated thatthe calibration status of the type of equipment is extremely poor, withsystems frequently operating outside of the permissible tolerances forthe ultrasonic power and/or ultrasonic intensity. A strong contributoryfactor to this is the fact that at the user level (physiotherapist)there are no simple tests by which the acoustic power delivered by thetreatment head can be verified in a traceable manner.

High quality (radiation) force balances exist, but the devices which arecommercially available are generally bulky, being built around atop-loading chemical balance and also require the input of a reasonablyskilled operator. The measurement systems currently available typicallycost from £1,500 to £2,500. Although simpler, deflecting vane type forcebalances exist and represent a cheap way of estimating output power,these measurements are not traceable and still remain too difficult toimplement at a user level.

These prior art techniques are thus too complex to provide the rapid,lower cost measurement capability required for implementation at theuser level.

The present invention seeks to provide means for measurement of thepower of such equipment and other ultrasonic emitters.

According to an aspect of the present invention, there is providedapparatus for measuring ultrasonic power emitted by a device asspecified in claim 1.

The apparatus of the preferred embodiment is able to measure totalultrasonic power delivered by a physiotherapy treatment head and canenable the performance of the physiotherapy device to be monitored on aregular, before use, basis and re-calibrated where necessary. In someembodiments, the system can measure effective radiating area of atransducer and/or local hot-spots in time-averaged acoustic intensity(beam non-uniformity).

The preferred embodiments can provide the potential for a compactmeasuring system which is lower cost and easier to use.

An embodiment of the present invention is described below, by way ofexample only, with reference to the accompanying drawing, in which thesole FIGURE is a schematic representation of a preferred embodiment ofmeasurement device, showing only the principal components thereof.

Referring to the FIGURE, the experimental set-up envisaged for thepreferred embodiment and the principle of operation are now described.In the FIGURE, there is provided a casing forming a chamber 16 which isopen at an upper side thereof. Within the chamber 16 there is provided apyroelectric element 12, which in this embodiment is in the form of athin membrane of thickness of around 0.040 mm or around 0.1 mm formed ofpolyvinylidene fluoride (pvdf). The film 12 overlies and is backed by anacoustical absorber layer 14 which, in this embodiment, is formed frompolyurethane rubber material.

In this embodiment, water is used in a well configuration 10 to overliethe film or membrane 12 and serves to couple an ultrasonic beam emittedby a treatment head 18 to the measuring sensor, namely the pvdf membrane10. Alternatively, instead of water another coupling member could beused such as a coupling gel. In either case, electrically insulatinglayer(s) should be provided between the film 12 and the water. These maybe formed from thin layers of plastic film (e.g. polyethylene) or rubbermaterial.

In the preferred embodiment the pvdf membrane 12 is a thin film toensure that acoustic reflections back to the face of the treatment headare small. If these are significant, the treatment head may change itsoutput power leading to error in the measurements. Very approximately,the thickness of the membrane 12 is preferably thinner that the acousticwavelength in water which is 1.5 mm at 1 MHz and 0.5 mm at 3 MHz.

In use, the incident ultrasonic beam will heat the material of theacoustical absorber 14 which is so designed that it absorbssubstantially all of the acoustic energy within a fraction of amillimetre of the interface between the membrane 12 and itself. Thepyroelectric film 12, in this embodiment the pvdf film, is used to sensethis increase in temperature.

The chamber 16 may include electronic circuitry 20 which can obtain anindication of the power being generated. Suitable circuitry will beimmediately apparent to the skilled person so is not described in detailherein. In one example, the circuitry 20 includes a d.c. peak detectionunit arranged to detect the peak value at switch on of the treatmenthead 18 under test, with the actual protocol for the measurements beingthe subject of routine experimentation. Measurements should only takeone or a few seconds.

The absorption coefficient of ultrasound in the acoustical absorber 14has been designed in the preferred embodiment such that the vastmajority of the ultrasonic power incident on the interface between thethin pvdf membrane 12 and absorber 14 is absorbed. This absorbedacoustic energy manifests itself as heat and, due to the pyroelectriceffect of the pvdf membrane 12, the resultant rate of temperature riseis measured as a d.c. voltage across the electrodes of the pvdf membrane12 which is measured.

The local rate of temperature rise is proportional to the time-averagedacoustic intensity and when integrated over the whole beam (which willbe the case when, in comparison to the size of the ultrasonic beam, thepvdf sensor area is large) this will provide a d.c. output which isproportional to the ultrasonic power being generated by the treatmenthead 18. A suitable display (not shown) can be provided to display theresults of the measurement.

In the preferred embodiment, the system measures the rate of temperaturerise by detecting a peak in output voltage from the membrane 12. Thispeak in temperature rise will typically occur very soon after switch onof the device 18 under test because subsequent to this, thermalconduction will cause the rate of temperature increase to decrease.

The acoustical absorber 14 plays an important role within the device. Atan acoustic frequency of 1 MHz, approximately 82% of the acoustic poweris absorbed within 1 mm of the of the front surface of the absorber 14.At 3 MHz, this increases to 99%.

The solid-state power meter design of the preferred embodiment is aimedat the physiotherapy ultrasound field. Acoustic powers generated by thistype of equipment generally lie within the range 0.5 watts to 15 wattsand, from the initial testing carried out, the device is sensitiveenough to measure powers as low as 100 mW.

The application range could be extended to measure diagnostic systemswhere the generated acoustic powers are lower (5 mW-200 mW).

The primary advantages of the described embodiment are as follows:

a) the system can be compact and lightweight, for example 100 mm outsidediameter and 40 mm deep;

b) the material costs can be relatively low;

c) the device can be very easy to use, with the ‘well’ shaped devicebeing filled with water, the face of the treatment head placed under thewater surface (roughly perpendicular to the top surface of the sensor)and the electrical drive to the treatment head switched on for a readingthen to be taken;

d) lateral alignment of the treatment head is not critical—it may bepositioned over any region of the pvdf membrane 12 and this shouldnominally produce substantially identical results and prevent theexcessive build-up of heat.

Many other variations are possible. For example, the transducer 18 maybe directly coupled without the water path 10 using coupling gel and canbe located above, within or below the absorber 14 whilst still directingits acoustic beam at the pvdf membrane. In principle, more complexelectrode patterns (consisting of, for example, multi-elements) could beused to obtain intensity distribution information within the beam.Clearly, this would result in a consequent increase in the complexity ofthe resultant instrument. For example, for the detection of effectiveradiating area (ERA) of a transducer 18 only part of the pvdf membranemay contribute to the output signal through the existence of a smallarea of highly absorber in contact with the member 12. This may be inthe shape of a circular disk of diameter 1 mm or so, which can then bemoved relative to the radiating area of the transducer 18 to determinethe intensity output profile and in particular power intensity over theradiating area. Alternatively, the member 12 could be formed by aplurality of elements, for example as a two-dimensional array ofdiscrete pyroelectric devices, as a series of strips or as a series ofconcentric rings. These alternative arrangements would allow theeffective radiating area of the transducer to be detected or localhot-spots to be detected.

The membrane 12 could be formed of other substances which exhibit apyroelectric effect, including piezoelectric material. Examples areceramic materials such as lithium niobate and quartz. The advantage ofpvdf, however, is that is deformable, is available in this layers ofthickness much less than the acoustic wavelength and is an excellentacoustic impedance match to water.

Similarly, the element 12 need not be in the form of a membrane and neednot be located above the absorber 14, that is between the absorber andthe device 18 under test. For example, the membrane could be locatedbelow the absorber 14, although this would result in loss of sensitivityof the member 12. In other embodiments, the member 12 could be locatedadjacent a wall of the chamber 16 or could form part of the wall. Inother embodiments, the member 12 could be a central element, for examplea strip located within the absorber 14.

The circuitry required to measure the voltage output may be separatefrom the chamber 16; a wide range of circuitry may be used including,but not restricted to, peak detection, integration, digitisation andsubsequent processing by computer or other means.

1. Apparatus for measuring ultrasonic power and/or intensity generatedby a device (18), including an element (12) including a material havinga pyroelectric effect, an ultrasonic absorber (14) adjacent the element(12) and in thermal contact therewith, and means (20) for measuringultrasonic power generated by a device (18) and absorbed by the absorber(14) using the pyroelectric effect of the element (12).
 2. Apparatusaccording to claim 1, wherein the element (12) is a thin film. 3.Apparatus according to claim 1 or 2, including a chamber (16) in whichthe absorber (14) is located.
 4. Apparatus according to claim 3, whereinthe element or membrane (12) overlies the absorber (14) in the chamber(16).
 5. Apparatus according to any preceding claim, including atransfer medium (10) operable to transfer ultrasonic power generated bya device (18) to the absorber (14).
 6. Apparatus according to claim 5,wherein the transfer medium (10) is water.
 7. Apparatus according to anypreceding claim, wherein the element or membrane (12) is formed frompolyvinylidene fluoride.
 8. Apparatus according to any one of claims 1to 6, wherein the element or membrane (12) is formed from apiezoelectric material.
 9. Apparatus according to any preceding claim,wherein the element or membrane has a thickness of substantially 0.04 to0.1 millimetres.
 10. Apparatus according to any preceding claim, whereinthe absorber is formed from a polyurethane material.
 11. Apparatusaccording to any preceding claim, wherein the absorber (14) is formedfrom a polyurethane rubber.